(a) Field of the Invention
The present invention relates to highly filled polypropylene compositions, to a novel calendering process using the compositions for production of polypropylene film (having a thickness of 250 .mu.m and under) or sheets (having a thickness of over 250 .mu.m), and to the polypropylene film or sheet produced from the process.
(b) Information Disclosure Statement
The calendering process was originally developed for the continuous manufacture of rubber sheet. It was later adapted to the manufacture of films and sheets of thermoplastics such as poly vinyl chloride (PVC), and, to a limited extent, polyolefins. Polyethylene films with filler content below 200 parts per hundred parts of resin by weight (PHR) have been commercially made by calendering process. However, polypropylene films and sheets are generally manufactured by processes other than calendering such as casting, blown film extrusion and flat-die extrusion with mineral filler CaCO.sub.3 content less than 100 PHR.
Various disclosures related to the manufacturing of polypropylene films and sheets by means of calendering have been made. Although production of smooth polypropylene sheets on calender has been achieved with limited success at low production rates and narrow web width, the surface smoothness decreases when the production rate and web width are increased. A major problem, which has stalled the progress of calendering of polypropylene, is that polypropylene tends to adhere persistently to the calender roll surface. Moreover, because it is more vulnerable to oxidative degradation and viscosity drop at elevated temperatures (i.e. greater than 210.degree. C.), no "bank" formation between the calender rolls occurs and polypropylene sheet formation is rendered impossible.
U.S. Pat. No. 2,462,331 discloses a calenderable, no filler added, polyethylene composition for hot-calendering on paper, fabric or other substrates. The lubricants used in this composition comprise a polyhydric alcohol ester of, or a metal salt of, a mono-carboxylic fatty acid having 12 to 24 carbon atoms.
U.S. Pat. No. 3,184,526 discloses a solution casting technique by calendering diluent-wet polymer for the production of self-supporting, no filler added, clear film. The polymer could be polyethylene or stereoregular polypropylene. Selecting proper diluent solvent and evaporating solvent from the extrudate are considered critical in this invention.
U.S. Pat. No. 3,328,503 discloses a process for forming oriented no-filler-added crystalline thermoplastic film and sheet which comprises passing a preformed web of molten amorphous polymer through the nip of a pair of counter-rotating rolls without forming a "bank" at the ingress side. The polymer can be polyethylene or polypropylene. A mixture of calcium stearate, stearic acid and dibutyl paracresol is disclosed as a calendering aid for polyethylene. This invention provides a method for orienting film through compression rather than tension. The absence of a "bank" at the entrance to the roll nip is considered a critical requirement in this invention. It is claimed that serious defects are formed which render the film useless when "bank" is present.
U.S. Pat. No. 3,660,551 discloses a method of improving the properties of calcium sulfite containing polyolefin sheet by virtue of inorganic acid treatment. Calcium sulfite existing near the surface of the sheet is effused and results in innumerable minute holes on the surface when the polyolefin sheet is treated with the acid solution. However, no calendering technique is taught in this prior art.
U.S. Pat. No. 3,993,718 relates a method for manufacturing an artificial paper requiring neither calendering nor biaxial orientation. The composition contains 30 to 94.8 wt. % of rigid polyolefin and 0.1 to 35 wt. % polystyrene and other ingredients.
U.S. Pat. No. 4,124,563 discloses a biaxial stretched molding sheet of a polyethylene composition containing 50-85 wt. % of calcium sulfite semi-hydrate. The specifically selected calcium sulfite is in the form of fine rectangular prismoidal crystals having a shorter axis of 1-30 .mu.m and a long axis of 5-100 .mu.m to allow a sufficient separation at the interfacial zone between calcium sulfite and polyethylene matrix when subjected to mechanical stresses. A method for the manufacture of the preferred calcium sulfite is also disclosed. However, neither the kneading and mixing technique for forming the composition nor the calendering and tenter stretching techniques for molding the void-possessed sheet are taught in this prior art.
U.S. Pat. No. 4,144,304 discloses a process for calendering polyolefins, including polyethylene and polypropylene, where the polyolefin is admixed with vegetable fibers. The presence of vegetable fibers prevents the mixture from sticking to the calender rolls.
U.S. Pat. No. 4,311,658 discloses a stripping process and apparatus for manufacturing thermoplastic, preferably polyvinyl chloride, sheets by calendering.
U.S. Pat. No. 4,345,046 discloses a lubricant composition for processing no-filler-added, polyolefin resins. The lubricant composition comprises a mixture of 60-70 wt. % of an N,N'-ethylene-bis-amide of a fatty acid or hydroxy-substituted fatty acid, 22-32 wt. % of a free fatty acid or hydroxy-substituted fatty acid, and 3-13 wt. % of a metal salt of a fatty acid or hydroxy-substituted fatty acid. The lubricant composition is particularly useful in the calendering of polypropylene homopolymers and copolymers when the processing temperature is around 385.degree. F. (i.e. 196.degree. C.). Polypropylene resins, which are preferred, have average molecular weights of about 80,000. However, calenderable, highly filled compositions are not taught in this prior art.
U.S. Pat. No. 4,615,853 discloses a method for producing thermoplastic sheet or filler-containing sheet by means of a pair of endless steel belts. A calendering method is not taught.
U.S. Pat. No. 4,695,239 relates to a calendering device following a sheet die (T-die) to produce no-filler-added sheets of thermoplastics having thickness between 100 .mu.m and 12 .mu.mm. Neither sheet extrusion nor calendering operation parameters are revealed in this art.
U.S. Pat. No. 5,063,005 relates to a process for preparing a heat shrinkable, no-filler-added film for pipe wraps by first forming a polyolefinic film--preferably polyethylene--on a three-roll calender. Thereafter, one surface of the resulting film is subjected to irradiation from a low energy electron beam. The technique of using the electron beam is taught in the disclosure. However, no calendering technique and operation parameters are disclosed.
Deutsche Patent No. DE3015-465 relates to polypropylene films with smooth surface, 40 to 100 .mu.m thick, prepared by calendering above crystalline melting point and at set speed difference of successive rollers of 1.1:1. The preferred composition comprises 70-95 wt. % polypropylene with a melt index (230.degree. C./21.6N) of 0.2-0.5 g/10 min and 5-30 wt. % polypropylene with a melt index (230.degree. C./21.6N) of greater than 7 g/10 min, or a mixture of 75-95 wt. % polypropylene with a melt index (230.degree. C./21.6N) of 0.2-0.5 g/10 min and 5-25 wt. % polyethylene having a melt index (190.degree. C./21.6N) of greater than 1.0 g/10 min. In any case, the resulting composition has a melt index less than 2 g/10 min which is relatively restrictive when compared to the present invention. The composition may also contain low levels of nucleating agents and/or fillers, and heat stabilizers. However, the no-filler-added composition is preferred. The temperature of four L-shaped calender rolls is set between 200.degree. C. and 225.degree. C. Therefore, this disclosure specifies an operating temperature which is only the lower portion of the present invention's range, and contains no processing aids, no matrix modifiers and little or no filler.
Japan Patent No. J47-3540 discloses a calendering process, which includes a separate heat treating roll, for no-filler-added polyolefin composition to give uniform smooth surface sheets. A polyolefin sheet is uniformly calendered at temperatures around 170.degree. C. After calendering, the film passes through treating rolls, which have a surface temperature of about 170.degree. C., to achieve a smooth surface. Neither plasticization of the polyolefin composition nor a calendering process for producing smooth film are taught in this prior art.
Japan Patent No. J49-5731 discloses a polyethylene composition for synthetic paper with thickness ranging from 0.04 mm to 0.5 mm. The polyethylene synthetic paper is made by initially kneading a polyethylene composition containing 90-40 PHR polyethylene, 10-60 PHR ethylene-vinyl acetate (EVA) copolymer, 100-200 PHR filler and 0.5-3.5 PHR sorbitan higher fatty acid ester on a two-roll mixer. Next, the composition is molded into a sheet using a calendering process. The preferred polyethylene has a melt flow index from 0.1-2.0 g/10 min and a density of 0.94-0.95 g/cm.sup.3. The preferred EVA contains 70-95 mol. % ethylene, melt flow index 0.5-20 g/10 min. The calendering technique and process of the present invention are not taught in this prior art.
Japan Patent No. J53-119946 discloses a calenderable polyolefin resin composition. The composition consists of 80-20 PHR of propylene-ethylene random copolymer containing 0.1-30 wt. % (0.1-5 wt. % preferred) ethylene and 20-80 PHR polypropylene homopolymer or propylene-ethylene block copolymer containing 0.1-5 wt. % ethylene content. The composition may further contain low loading level of colorant, heat stabilizers, lubricants, fillers, etc. However, none of the examples shown in this art contains filler in its composition. Neither plasticization of the polyolefin composition nor a calendering process are taught in this prior art.
Japan Patent No. J62-010141 discloses a porous sheet or film production method. The sheet or film is prepared by T-die extruding and stretching at a draw ratio of greater than 1.5. The film composition contains polyolefin resin such as linear low density polyethylene and high density polyethylene, filler and triglyceride. This process provides white film or sheet having continuous pores by drawing film at ordinary temperature. The resulted sheet or film can be used for clothing, medical material, hygienic material, filter, etc. This disclosure, however, does not teach a calendering process.
Japan Patent No. J62-027438 describes the formation of breathable film by uniaxially stretching a film having 42 to 87 vol. % polyolefin resin and 13 to 58 vol. % of inorganic filler, e.g., calcium carbonate. A calendering technique is not taught herein, and very fine filler (0.6 to 3 microns) is required.